There are many situations in which there is a need to measure a magnetic field. Among such situations are the measurement of position or proximity of a magnetized portion of a structure, the reactant of stored magnetic information, the measurement of current flows without the need of a measuring device in the current flow path, etc.
Many of the magnetic effects in such situations are relatively small and therefore require a sensitive magnetic sensor. A magnetic sensor capable of sensing such small magnetic field perturbations, and which is economical to fabricate, is provided on the basis of the magnetoresistive effect. Such magnetoresistive material based magnetic sensors can be fabricated as thin films when using monolithic integrated circuit fabrication techniques, and so can not only be made economically but also made quite small in size. A magnetoresistive material based magnetic sensor is arranged by providing a magnetoresistive material to be used as an electrical resistor. A current is passed therethrough, and the voltage there across will depend on the effective resistance of the material over the path in which the current flows. That resistance value will depend in turn on the state of the magnetization of the material. If the magnetization is parallel to the current flow, what is the case for Anisotropic Magnetoresistance (AMR), the material will exhibit a maximum resistance, and it will exhibit a minimum resistance for magnetization perpendicular to the current flow.
For Giant Magnetoresistance (GMR), the maximum resistance is for parallel alignment of the magnetization of adjacent magnetic layers, separated by non-magnetic interface layers. A spin valve system consists of two magnetic layers, a free layer and a pinned layer. The pinning can be made by an antiferromagnetic layer or by antiferromagnetically coupled pinning layers.
The current in such systems can be current-in-plane (CIP) or current-perpendicular-to-plane (CPP). The CPP structure is normally used in tunneling devices (Tunneling Magnetoresistance—TMR), where the non-magnetic interface layer consists of an electrically resistive isolator material.
In the magnetoresistive device there is typically a free rotating layer with an effective magnetization. An external field acting on the magnetoresistive material will rotate the magnetization direction therein to change the resistance of the layer system as a result. The changed resistance of the material carrying the current causes a voltage drop change across the resistor which can be sensed as an indication of the magnitude of the external field.
The effective resistance of such a film will vary as the square of the cosine of the angle between the effective magnetization direction and the current flow direction through the material in the AMR case and as the cosine of the angle of adjacent layers in the GMR or TMR case. The total resistance, however, is usually not of interest but rather the change in resistance in response to a change in the applied external magnetic field. In the AMR case, this change is often best measured at a point along the squared cosine response curve where the curve approximates a linear function.
To provide operation on such a linear portion of the response curve requires that there be an initial angle between the direction of current flow and the nominal direction of magnetization in the absence of any externally applied fields. This can be accomplished in alternative ways in a bias arrangement. The magnetoresistive material can be placed on the device substrate as a continuous resistor in a “herringbone” pattern or other design of continuously connected multiple inclines, with the angle of incline being approximately 45° with respect to the direction of extension of the resistor. There then must be provided a source for a magnetic bias field which is oriented in a direction which is 90° to the direction of the extension of the resistor.
Another method is to provide a linear strip of magnetoresistive material, with additional individual conductors across that strip at an angle of 45° with respect to the direction of the strip. This, in effect, causes the current to flow at an angle through the magnetoresistive strip with respect to the direction of elongation of the strip itself. This latter configuration is often called a “barber pole” sensor because of its configuration, and such an arrangement can eliminate the need for an external source of a magnetic bias field.
In magnetic recording heads the magnetization of the sensing layer of an AMR sensor is rotated by 45° in relation to the sense current by the stray field of an adjacent magnetic layer magnetized perpendicular to the direction of the sensor strip. This layer can be a hard magnetic material (hard bias layer) or a soft magnetic material (soft adjacent layer) magnetized by the sense current.
In GMR or TMR elements the magnetization of the free layer has to be directed parallel to the strip direction. This is normally done by a hard bias layer placed on each side of the sensor. The magnetization of the pinned layers will be fixed perpendicular to the strip direction by antiferromagnetic coupling.
Magnetostriction is an essential parameter for controlling the magnetic properties of thin films and multilayers. Magnetostriction describes the change in length of a magnetic material by magnetic reversal.
In magnetic recording elements it is important to have homogeneously magnetized magnetic layers, especially the sensing layer (free layer) in the sensing layer stack. Inhomogeneously magnetized regions, like vortices or magnetic domains, cause instabilities in the recording signal. Therefore, the magnetic layers are aligned by local magnetic fields (exchange coupling field, hard bias field). Local inhomogeneities can be caused by magnetostrictive anisotropy. Therefore, the magnetostriction has to be controlled very precisely.
Various experimental methods have been developed for investigating the magnetoelastic properties of thin films. One of them is the direct measurement by the so-called “cantilever method”. A change in magnetization leads to a change in length which with thin films causes bending of the substrate. This is, e.g., described in E. du Trémolet de Lacheisserie et al., “Magnetostriction and internal stresses in thin films: the cantilever method revisited”, Journal of Magnetism and Magnetic Materials 136 (1994), pp. 189-196.
Another possibility is the indirect measurement by means of the strain gauge method, which creates mechanical stresses in a magnetic film. The magnetic anisotropy changes through magnetostrictive coupling. This is, e.g., described in D. Markham et al., “Magnetostrictive measurement of magnetostriction in Permalloy”, IEEE Transactions on Magnetics, vol. 25, no. 5, September 1989, pp. 4198-4200.
An apparatus for measuring the magnetostriction constant of a magnetic membrane is disclosed in Patent Abstracts of Japan, JP 62106382 A2.
Kenji Narita et al., IEEE Transactions on Magnetics, vol. Mag-16, no. 2, March 1980, pp. 435-439, disclose a method to measure the saturation magnetostriction of a thin amorphous ribbon by means of Small-Angle Magnetization Rotation (SAMR).
However, no method is known to measure the magnetization changes using the magnetoresistive effect of magnetic sensors directly, so that the real environment of the sensor is reflected. Therefore, there is still a need for improvement of such methods.